1. Field of the Invention
This invention relates to a process for forming a titanium nitride layer on a semiconductor wafer by annealing a titanium layer, already deposited on the wafer, in a nitrogen-bearing atmosphere under reaction conditions which favor formation of titanium nitride.
2. Description of the Related Art
In the conventional formation of a titanium silicide layer, as a part of an integrated circuit structure on a semiconductor wafer such as a silicon wafer, a titanium layer is first deposited over a silicon semiconductor wafer. The silicide is then formed during an anneal by the reaction between the titanium and underlying exposed silicon regions of the wafer. The reaction is carried out in a sealed chamber in the presence of nitrogen gas which results in the simultaneous formation of a surface layer of titanium nitride over the titanium silicide which serves as a barrier layer to prevent migration of unreacted silicon atoms through the titanium silicide layer to the surface.
The silicide-forming anneal is usually carried out at a temperature within a range of from about 650.degree. C. to about 695.degree. C., at which temperature range, formation of titanium silicide will readily occur. Higher temperatures are usually avoided because those portions of the titanium layer overlying silicon oxide (SiO.sub.2) regions of the semiconductor wafer may react with the silicon oxide to form both titanium oxide and titanium silicide, the presence of neither of which is desirable over the silicon oxide (insulation or isolation) regions of the wafer.
Temperatures below about 650.degree. C. have usually been avoided in the prior art because the presence of oxides on the titanium usually inhibited formation of either the titanium silicide or the barrier layer of titanium nitride unless a temperature of at least 650.degree. C. was used. In particular, the presence of oxides usually inhibited the adequate penetration of nitrogen into the layer to form the desired titanium nitride blocking layer required to inhibit silicon migration to the surface. This can be seen in the Rutherford Backscattering Profile of FIG. 1 which illustrates the inadequate nitrogen penetration when a prior art silicide anneal process was carried out at 625.degree. C. after the titanium surface had been exposed to air.
Such oxides are conventionally present due to the prior art practice of conventionally depositing a layer of titanium over a cleaned wafer structure in a vacuum deposition chamber, for example, using a vacuum sputtering deposition (PVD) process, to deposit about 100 to about 1000 Angstroms of titanium on the wafer, and then removing the titanium-coated wafer from the deposition chamber and transporting it through the ambient atmosphere to separate annealing apparatus, resulting in the pick-up of oxygen and/or oxygen-bearing gases, such as air, on the surface of the newly deposited titanium layer.
In addition, oxides might also be present in prior art practices due to inadequate wafer cleaning practices. Although those surfaces of the wafer on which the titanium silicide layer was to be formed were first cleaned in prior art practices to remove any materials which might interfere with reaction between the subsequently deposited titanium layer and the exposed silicon portions of the wafer, the conventional cleaning was usually carried out in a vacuum chamber using an inert gas such as argon with an rf plasma (either with or without a previous oxide wet etch), after which the cleaned wafer was usually transported through the ambient atmosphere to the deposition chamber. This exposure of the cleaned wafer to oxygen-bearing gases, as well as the inadequacies of the inert gas/rf plasma cleaning, provided additional sources of oxides and other impurities on the wafer surface.
It would, however, be desirable to provide a process for the annealing of a titanium layer on a semiconductor wafer under conditions where penetration of nitrogen into the titanium layer would occur, even while annealing at a temperature at which the formation of titanium nitride instead of titanium silicide would be favored.